The electronic surveillance art has seen continued advance in the past decade in both sensitivity and consistency of detection of suitably tagged articles in unauthorized transport thereof from secured areas.
One significant advance is disclosed in commonly assigned U.S. Pat. No. 3,895,368 issued on July 15, 1975 to Gordon et al., wherein joint high and modulated low frequency fields, respectively radiant electromagnetic and electrostatic, are established in the surveillance or control zone of an area also including an article display zone. Articles of concern have receptor-reradiator devices secured thereto and such devices have capability for transmitting an output signal having preselected correspondence to energy incident thereon.
In the Gordon et al. systems, the high-frequency field results from a continuous wave (CW) microwave frequency transmitter, e.g., operating at nine hundred and fifteen megahertz, and the low frequency field results from a transmitter operating at a substantially lower frequency with a time-based, frequency-modulation (f-m) characteristic. An alarm or other indication of unauthorized article presence in the surveillance zone is provided when a receiver receives a receptor-reradiator output signal transmission which includes content corresponding to the high and low frequencies and detects the modulation characteristic of the latter. Output alarm indication is preferably delayed in the '368 patent until there is a given repetitive occurrence of receipt of the proper detected alarm signal.
Alternative effective alarm condition sensing is achieved in another commonly assigned patent, U.S. Pat. No. 4,139,844 issued to Reeder on Feb. 13, 1979, wherein different modulated low-frequency signals are propogated with the microwave energy in the surveillance zone. Here, an alarm condition occurs only where a signal is detected which has content corresponding to a composite characteristic of the plural low frequency signals.
Fundamental to these systems of the assignee hereof is a recognition of need for a high-frequency carrier, the microwave transmission, and of the inherent difficulties of limiting the transmission pattern thereof, and of the detectable alarm intelligence therewith, through modulation, of a low-frequency carrier whose propagation pattern is comparatively controllable. Control of the pattern of the low frequency field is thus practical and gives rise to some measure of distinction as between the surveillance zone and an adjacent article display zone in which tagged articles are to be present without creating an alarm condition.
A phenomenon of wave propagation is that pattern control is inexact and a so-called "over-range" characteristic is inherent, i.e., patterns will extend beyond anticipated or planned ranges, dependent upon various factors, such as the presence of pattern-influencing extraneous objects. Mechanical shielding, e.g., conductive screens, have provided a degree of limitation on over-range, but are generally undesirable, based on physical size, aesthetic requirements and the impracticality of physically shielding large areas. In the absence of such screening, tagged articles outside of the surveillance zone can give rise to alarm indication and full usage of an article display area may accordingly be diminished.
One known approach to overrange control is shown in U.S. Pat. No. 4,212,002, issued to Williamson on July 8, 1980. In the system of the '002 patent, a high-frequency field is established in an area and a first low-frequency field with a modulation characteristic is established in a surveillance zone. A second low-frequency field with a modulation characteristic is established adjacent the boundary of the surveillance zone by antenna means transmitting energy directionally away from the surveillance zone. The frequency of the second field signal is distinct from that of the first field, such that a frequency-modulation receiver at the surveillance zone can discriminate between receptor-reradiator output transmissions from articles in the surveillance zone and in the boundary zone. The operative mechanism is the f-m receiver's "locking on" to the strongest of the different frequency carriers within its pass band. Transmitters of articles in the adjacent boundary zone will provide output signals having the second field carrier dominating over the first field carrier, and accordingly will not cause alarm indication. Upon receiving such transmissions from the boundary zone, the receiver operates in its customary mode, i.e., detecting the modulation characteristic of the second frequency carrier. Output may accordingly be provided, advising of the approach of an article toward the surveillance zone. In applicants' view, the operative mechanism of the '002 system, namely, the need to lock on to plural carrier frequencies, can give rise to a lessening of sensitivity of the receiver to transmissions issuing in the surveillance zone.